Circuit breakers provide a means for controlling supply of electrical power to a circuit, which may be a single-phase circuit, a three-phase circuit, or a multi-phase circuit with a switched neutral, for example. A trip unit, such as an electronic trip unit control module, is commonly used to interface with the breaker to control tripping characteristics such as, rate of tripping and trip current, for example. Trip units may be removable from the breaker for servicing and for replacement by trip units having alternate tripping characteristics. A breaker being in a closed configuration while the trip unit is removed from the breaker may be an undesirable condition since the circuit may not be trip protected without the trip unit being installed. The industry may, therefore, be desirous of a system to interlock the trip unit with the breaker.